This invention relates to smoke detectors in which a radioactive substance is used in conjunction with one or more ionization chambers. Smoke detectors of this kind, which are well known, include a first electrode and a second electrode defining between them an ionization chamber adapted to allow smoke to enter from the surrounding atmosphere. A radioactive substance emits radiation into the ionization chamber to cause ionization of gas therein. A potential difference is maintained across the two electrodes so as to cause an ion current to flow between them which current is affected by entry of smoke into the ionization chamber. The alteration in ion current can be detected and caused to trigger an alarm.
Detection of this alteration is a problem, and various devices have been proposed to solve it. One device involves division of the chamber into two ionization regions arranged so that the entry of smoke affects the ion current in one but not the other. If one electrode is made common to both ionization regions, then an alteration in one region but not the other alters the potential of the common electrode and this alteration can be detected.
British Standard 5446: Part 1: 1977 provides that smoke detectors intended for use in residential premises must be capable of being tested. Testing without the help of smoke of standard density is somewhat tricky. One device that has been used is a shield which can be moved in front of the radioactive material, but this requires a moving part in an inaccessible place which is somewhat expensive and unsatisfactory. The present invention enables any smoke detector employing the above principles to be tested to ensure correct functioning without requiring any mechanically moving parts in the detector.